In order to improve the efficiency of the Otto cycle one must increase the compression ratio of an internal-combustion engine. However, in the gasoline engine there is a limit – the gasoline-air mixture will self ignite once the compression gets too high (because every compression drives temperature increase). So, either you can have a low-efficient, low-compression engine that uses a cheap fuel, or you can have a high-efficient, high-compression engine that uses expensive, high-refined fuel that wont self-ignite even at high compression levels (a 120 octane gasoline?).

In diesel engine this problem is solved. The diesel engine can use much higher compression levels than the gasoline engine reaching higher efficiency. In addition, the diesel engine can use fuel that is not nearly as refined as the high-octane gasoline fuel (thus cheaper).

Of course the Carnot cycle is an ideal cycle and indicates the upper value of achievable performance.